Equality of feeding roles and the maintenance of monogamy in tree swallows

Summary We investigated the division of labor in male and female tree swallows by measuring the rate of food delivery to nestlings at 36 nests. By observing natural nests and performing brood manipulations we found that males and females divided the feeding duties about equally and responded similarly to changes in brood size and age. Feeding rate was most highly correlated with brood mass. Manipulation and removal experiments showed that increased feeding rates could be elicited, but only for limited periods of time. Male and female tree swallows could only partially compensate in feeding nestlings when mates were removed. This, along with the higher mortality in enlarged broods and in those raised by single parents, indicates that both male and female are required to raise an entire brood to fledging. We argue that this requirement contributes to the absence of mate guarding and the maintenance of monogamy in the tree swallow.